1. Field of the Invention
This invention concerns a process and a device for securing documents. It envisages, in particular, identifying a document in a unique way, authenticating it, i.e. being able to detect its copying and/or carrying, on the document, information relative to this document, for example information identifying an owner of intellectual property rights connected to the document and/or its place of manufacture. The term document includes all data carriers, for example hardcopy documents, blueprints, packaging, manufactured items, molded items and cards, e.g. identification cards or bankcards.
2. Description of the Related Art
The different types of document printing are divided into two groups: one known as “static”, in which each document receives noticeably the same printed mark, for example an “offset” analog print process, and the second known as “serialized” digital, in which each document receives an individualized item of information, for example an ink-jet print process controlled by an individualization program, and a process for printing a serial number.
For offset printing, which is one of the most commonly used print methods for boxes and packaging, a plate is generated for each color printed in the document, and this plate's content is printed hundreds of thousands, even millions, of times. In this case, the same content, inserted on the printing plate, is printed on every document for every print. Flexography, typography and gravure printing are other examples of what are called static printing methods. In static printing documents cannot be identified individually, in theory, since the same mark is printed each time. In addition, when the printing is static and makes use of analog processes, it is more difficult to control the exact number of documents printed. The risks of counterfeiting through printing a larger quantity of documents than the owner of the rights has authorized are therefore significant. How can you ensure that the number of prints specified by the manufacturing order, often less than the plate's usage limit, has been respected? How can you ensure that all the unused prints (start or end of the series, faults, order cancelled, etc) and all the plates, films and other objects that allow the documents to be reconstituted never fall into the hands of counterfeiters?
Serialized printing, by allowing each document to be precisely and unequivocally identified, is generally preferable to static printing. In effect, each identifier being only printed once in serialized printing, reading a double means that an alarm can be triggered: a double is an identifier that is identical to a previously read identifier.
In a general way, there are several points to be made secure in order to protect identifier and/or anti-copying marks: the source file, possibly the CAP file that contains it, and, in the case of offset printing, the plates and any films.
It is possible to perform the equivalent of serialized printing of an anti-copying mark on an item already printed statically by, in a second step, printing a unique code or serial number that is uncoded or, preferably, encrypted. This serialized printing can, for example, take the form of a two-dimensional bar code. Outwardly, this procedure makes it possible to track each document individually and at the same time retain a sure way of detecting copies. Stolen documents that have not received the serialized print would not bear a valid identifier.
This approach does not, however, solve all the problems. In effect, while a wrongdoer cannot identify the falsified documents as the printer would have done, the unique code printed by the serialization printer, generally offering a limited print quality, is not protected against copying.
Counterfeiters having in their possession documents to be identified as authentic can therefore copy one or more valid unique codes and re-copy them onto documents to be identified as authentic.
The prior state of the art contains several methods exploiting measurable physical characteristics in order to characterize and identify each document in a unique way. In general, the measurable physical characteristics chosen are of a random nature, and according to the actual state of the art and technologies cannot be copied, at least not in a cost-effective way. These methods enable all the documents considered “valid” to be controlled: only those documents for which the physical characteristics, comprising a unique set, have been memorized are considered valid.
For example, U.S. Pat. No. 4,423,415 describes a method enabling a sheet of paper to be identified according to its local transparency characteristics. Several other procedures are based on inputting unique and non-reproducible physical attributes of the material in order to generate a unique and non-transferable signature of said document. For example, documents WO 2006 016114 and US 2006/104103 are based on the measurement of the diffraction pattern induced by a laser ray applied to a precise area of the object.
Although they offer an interesting solution to the problems mentioned above, the approaches based on extracting a signature from the material are difficult to use for a number of reasons. Firstly, recording signatures when the documents are produced requires a costly optical reader and is difficult to integrate into production lines. These latter may, moreover, have very high working speeds. In a general way, it seems that these techniques are only applicable to small-scale production. In addition, the reader used for checking, in the field, is also too costly for a number of applications. It is also bulky and not easy to use, while often the checks in the field must be done rapidly and unobtrusively. Finally, it is not possible to extract a unique signature for all materials: glass and objects that are too reflective are excluded, in particular, at least for measurements of a laser's diffraction.
This invention aims to remedy these inconveniences and in particular the difficulties and limitations of applying known identification methods based on the unique physical attributes of the document's matter.
The digital authentication codes, also called “DAC” below, are digital images that, once marked on a medium, for example by printing or local modification of the medium, are designed so that some of their characteristics, generally automatically measurable from a captured image, are modified if a marked image is copied. The digital authentication codes are generally based on the degradation of one or more signals sensitive to copying during the copy step, a signal being borne by image elements with measurable characteristics sensitive to copying. Certain types of digital authentication codes can also contain information allowing the document containing it to be identified or tracked.
There are several types of digital authentication codes. The copy detection patterns, also called “CDP” below, are dense images, generally of a pseudo-random nature. Their reading principle is based on an image comparison in order to measure an index of similarity (or dissimilarity) between the original copy detection pattern and the copy detection pattern captured, for example by an image sensor: if this captured image is a copy it will have a lower index of similarity than if it is an original.
Like the two-dimensional bar codes, the secured information matrices, also called “SIM” below, are images designed to carry a large quantity of information in a robust way. However, unlike two-dimensional bar codes, secured information matrices are sensitive to copying. On reading, an error rate is measured for the coded message extracted from the matrix, a rate that is higher for the copies than the originals, which allows these copies to be distinguished from original prints.
Unless marked in a special way, for example with invisible ink, the copy detection patterns and secured information matrices are visible. In addition, marking the copy detection patterns and secured information matrices in an invisible way is not always possible, due to cost or manufacturing constraints. The visibility of an anti-copying mark can be a disadvantage in terms of esthetics and, in certain cases, security since the counterfeiter is informed of their presence.
There are also digital authentication codes that are naturally invisible or at least difficult to see.
For example, some digital marks (known under the name “watermarks”) integrated into printed images are designed so as to be damaged when the printed image is reproduced, for example by photocopying. The measurement of the digital watermark's degree of deterioration, lower in the original print than in a copy of it, makes it possible to detect these copies.
The combination of several watermarks with different degrees of sensitivity to copying makes it possible, by comparing the respective energy levels, to detect the copies. Integrating digital watermarks in the production procedures of documents is, however, more complex, which limits their use: in effect, unlike copy detection patterns and secured information matrices, the digital watermark cannot be simply “added” to the image; the digital watermark is, in fact, a complex function of the message to be added and of the original image, the digital watermark's energy being locally adjusted according to the original image's masking properties. Integrating digital watermarks in documents or products entails sending the source image to a marking/printing central processing unit that integrates the digital watermark and sends back a marked image. This procedure is not very practical, because of the often large size of the files and related image security problems. In contrast, for marking/printing with a copy detection pattern or secured information matrix, the source image does not have to be sent to the marking/printing central processing unit: conversely, it is the image of the copy detection pattern or secured information matrix, generally of a small size, for example several kilobytes, that is sent to the holder of the image files that will be affixed onto the document or product. In addition, it is very difficult to stabilize the reading of digital watermarks, which makes the determination of the copy from the original of a document more random. In effect, the risks of error are generally noticeably higher with digital watermarks than with copy detection patterns and secured information matrices.
There are also asymmetric modulation spatial marking processes, also called “AMSM” below, such as those described in documents WO 2006 087351 and CH 694 233. Just like digital watermarks, AMSMs allow documents to be marked invisibly, or at least unobtrusively. AMSMs are generally patterns of dots, which are added as an additional layer to the document to be marked. For example, in the case of an offset print process, an additional plate bearing only the AMSMs is overprinted on the document. In this way, the AMSMs are more easily integrated than digital watermarks into the document production process, the source image not being required by the marking/printing central processing unit. However, unlike copy detection patterns and secured information matrices, the AMSMs generally require an additional plate and ink, which makes their use more complex and more costly. In addition, just like digital watermarks the AMSM detection methods can be imprecise. In fact, it is known that the marking/printing entails an analog uncertainty concerning the precise positioning of the marked image. This uncertainty, at the level of the dimension of the printed elementary dot, even below this, has a not insignificant effect on the detection of copies when the surface marked has a significant size. However, AMSM detection methods, based on auto-correlation and cross-correlation, cannot take this uncertainty of position into account. This increases the imprecision in reading the mark and, as a consequence, reduces the ability to distinguish between the originals and the copies.
When the capture is done by flat-bed scanners, allowing both a large capture surface and a sufficient capture resolution, the AMSMs enable simple copies to be detected, for example photocopies, even high-quality photocopies done by capture with a high-precision or high-resolution scanner, followed by reprinting. Nevertheless, in the face of a determined counterfeiter, AMSMs offer reduced protection against copying. In effect, after the high-resolution capture the counterfeiter can use manual image processing tools, such as “Photoshop” (registered trademark), possibly combined with automatic image processing tools (such as “Matlab”, registered trademark), in order to restore all the detected dots in their initial form. In the case of a high-quality copy, the dots will no longer be weaker in the copied mark than in the original mark, and the copy has a strong chance of not being detected as a copy. Thus, a determined counterfeiter can generally make an identical copy of the information contained in an AMSM, which means that this method cannot be considered secure in the long term.
For the most commonly used print methods (in particular offset), the AMSMs (and other digital authentication codes) are printed statically. As the types of printing most commonly used for AMSMs and digital authentication codes are static, it is not possible to vary the mark and the contained message on each print.
Nevertheless, it may be desirable to be able to uniquely characterize, and thus identify, each print from a single source image. Similarly, it would be desirable to identify the printing plate that was used to print a document, so that these documents can be traced.